Listening to Irish architects Yvonne Farrell and Shelley McNamara speaking about their work, you could be forgiven for thinking they were describing a dramatic mountain range, or the rocky coasts of County Clare. They talk of cliff faces, summits and ravines, of spaces being carved and cleft to form grottos, overhangs and terraces. When you encounter their buildings, it becomes apparent why: they feel like structures that have been wrestled into shape with geological force.

The result of their latest tectonic rupture was recently crowned “best new building in the world”, having been awarded the inaugural RIBA international prize in December. This was a suitably momentous accolade for a vast concrete monolith that doesn’t pull its punches. Perched dramatically on the edge of a ravine that rises above a motorway in Peru, the Universidad de Ingeniería y Tecnología (UTEC) is a symphony of soaring buttresses and daring cantilevers, an apt home for Lima’s new university of engineering, where students will learn the art of mining minerals from the Earth’s crust.

“We’re interested in weight,” says McNamara, 64, who founded Grafton Architects with Farrell, 65, as a co-operative in 1978. “For us, the enjoyment in architecture is the sense of weight being borne down or supported, the feeling of moving within the forces of gravity. It’s a very primal need.”

It’s certainly a feeling that will be impressed upon the students walking into UTEC on their first day of term. Great structural fins march along the spine of the building at 20-metre intervals, supporting a vertiginous stack of laboratories and classrooms, rising up either side of a nave-like void that runs through the centre of the complex. Looking up, you get a thrilling view of intersecting concrete beams and slabs, an aerial ballet of staggered terraces connected by flying walkways and leaping staircases. Given the amenable climate, it is all open to the elements, allowing coastal breezes to flow through the atrium, and the sounds and smells of the street to bleed into the building.

“It’s an extension of the city,” says Farrell, describing how most people in Lima still build their own homes, pouring the concrete themselves (more cement is sold on the domestic market in Peru than in the commercial one). The result is a city of densely terraced, stepping rooftops clinging to the hillsides. “We needed to make something at a heroic scale,” she adds, “but which also had intimacy and a range of smaller spaces where students could retreat.”

Within this mountain of infrastructure, which looks like a fragment of an ancient colosseum, there are little nooks and patio gardens, sheltered places to be alone and enjoy the view out over the city. Drawing on the ancient terraced landscape tradition of Peru, they have crafted a modern-day Machu Picchu.

Located in the Barranco district, the building acts as a buffer between the busy highway and a quiet residential area, forming an urban cliff that will eventually extend 360 metres along (the completed building is the first of three phases). In its uncompromisingly muscular form, it might feel like a bit of a throwback to an earlier, brutalist era. There are definite echoes of the Alexandra Road estate in north London, designed by Neave Brown in the 1960s, and the stepped concrete fortress of the Brunswick Centre in Bloomsbury. Proposing this kind of megastructure might suggest the sort of dogmatic self-assurance of postwar planning, but Farrell and McNamara have no“starchitect” hubris. They are unusual in that they don’t present a project as a fait accompli, or the inevitable “right answer” to a brief or site.

Rather than thinking of a space and then finding a structre for it, we make a structure and that, in turn, makes a space

“We never really know what we’re doing when we start,” McNamara admits. “We like to create spaces you couldn’t design consciously, things that just happen somehow. Rather than thinking of a space and then finding a structure for it, we make a structure and that, in turn, makes a space.” They say they are interested in “the spaces in between, which haven’t been asked for in the brief” and describe their architecture as a kind of “scaffolding”, a non-prescriptive framework on which lives and events can be played out.

UTEC feels like a gigantic armature with the teaching spaces slotted in, a sense that gives it an ancient, almost ruined quality. Over time, its grey concrete bones will accrue a layer of the surrounding desert dust, lending it the warm earthy hue of the city’s older concrete structures, and adding to the primal feel.

The RIBA prize may have put Grafton in the spotlight, but they are no strangers to international gongs. They won the inaugural World building of the year award in 2008 for their Universita Luigi Bocconi building in Milan, clad in sheer walls of stone that appear to slip and slide past each other, while their sharply chiselled medical school and student accommodation for the University of Limerick was shortlisted for the Stirling prize in 2013.

Grafton’s tectonic power will soon be coming to London, with two substantial buildings currently in the works. Toning down the Richter scale to a level palatable for leafy suburbs, their Town House for Kingston University will be a handsome civic addition to the campus, providing teaching spaces and cafes arranged on cascading terraces around a central courtyard, with an open colonnade to the street to invite in the public.

Eschewing raw concrete for a polite palette of reconstituted stone and brick, in a nod to nearby Hampton Court Palace, the building will nonetheless share UTEC’s feeling of an open framework to encourage a fluid overlapping of activities. It was initially refused planning – likened to a “second world war flak tower” by the council planning committee – but further explanatory drawings thankfully convinced them there was nothing to be afraid of.

Taking inspiration from the atmospheric fan-vaulted undercroft of the 1600s Lincoln’s Inn Chapel, Grafton Architects have proposed an internal structure that appears to grow in branching, tree-like umbrellas as it rises through the building. Spiralling staircases will corkscrew down the multiple levels, landing on a gently sloping open plaza, conceived as an extension of the public realm outside. The pair describe the space as having a similar quality to the Royal Festival Hall or the British Library, “acoustically as well as visually”, again revealing their attention to crafting spaces with an awareness of all the senses.

“It feels like a monastery,” says Farrell of the building. “A place where there’s more time for conversations about how each place is unique – and why architecture’s role is to heighten that, not to smother it.”